A trough usually contains a few dozen cables. These cables may include ones, the service lives thereof having expired due to deterioration with time or ones that have incurred failures and are not serviceable. In such a situation, when a new cable or cables are to be laid in the trough, it frequently happens that the new cable cannot be laid unless such unusable cables are removed from the trough.
When laying a new cable, however, a worker may not be able to identify which cable or cables in the trough are necessary and which one or ones are unnecessary. For this reason, there is a marked increase in the number of cases where workers are unable to remove unnecessary cables from troughs. In addition, accidents wherein workers erroneously remove cables in service when laying new cables have occurred. Therefore, in this technical field, there has been a demand for an apparatus which enables workers to open the cover of a trough in a desired place and locate a particular cable.
In the past, in response to such a demand, there was a method whereby a cable was identified by applying a 100VAC voltage to a conductor of the cable to be removed and detecting the voltage by using a commercially available electroscope. However, a 100VAC voltage is constantly applied to most cables in troughs. In this method, therefore, a plurality of cables occasionally react at the same time, making it difficult to identify cables. Furthermore, there is such a danger that a worker may get an electric shock from touching a cable to which the 100VAC voltage is applied.
So, as another identifying method, there was a method wherein a prescribed current is let flow through a particular cable to be removed, the cable in the trough is clamped with a cable probe consisting of a search coil, and the current flowing through the cable is detected using an electromagnetic induction method, thereby identifying the particular cable.
This identifying method, however, presented a problem in that a circuit such as a common grounded circuit for letting currents flow through a cable to be removed was required. The identifying method had another problem in that its cable identifying reliability was low because it was susceptible to noises in its surroundings. There was still another problem in that if the identifying method was used for a portable apparatus, dry cells for causing currents to flow through cables were exhausted quickly. Moreover, according to the identifying method, cables must be clamped and checked one by one using the search coil to locate a particular cable among a plurality of cables by increasing sensitivity, thus resulting in poor work efficiency.